The C-nucleosides are a class of natural products which exhibit remarkable antibiotic, antiviral, and antitumor activity. A research program is proposed which will focus on the development of both a totally synthetic route to these products and their isosteres as well as a semi-synthetic approach to analogues of the "hypermodified nucleosides." For the total synthesis approach, a heterolytic C-C bond scission of an oxabicycloheptane derivative serves as the key step in the construction of functionalized tetrasubstituted tetrahydrofurans which are then elaborated by standard methods to the target molecules. All new compounds will be submitted for biological screening as antitumor agents so that structure-activity relationships can be established.